In a gas turbine engine, compressed air is mixed with fuel in a combustor and ignited for generating hot combustion gases which flow downstream through one or more stages of turbine vanes and blades from which energy is extracted for producing output power. The vanes, blades, and flowpath bounding the combustion gases are therefore subject to substantial heat load, and are therefore typically cooled for obtaining a suitable useful life thereof.
In order to protect hot flowpath components such as vanes, blades, and turbine shrouds, they may be coated with a thermal barrier coating (TBC), such as ceramic zirconia for example, which provides an effective thermally insulating barrier between the base metal of the components and the hot combustion gases.
Thermal barrier coatings are typically applied to the base metal of the various components by spraying the TBC in powder form, melting the powder, and coating the component with the molten powder is a series of overlapping layers which solidify on contact with the component to produce a solidified thermal barrier coating of appropriate thickness.
A typical process uses a conventional plasma torch which produces a hot ionized plasma for melting the TBC powder injected therein which is then sprayed atop a conventional bondcoat on the workpiece. A typical TBC is yttria stabilized zirconia which has a melting temperature of about 2,700.degree. C. The plasma torch and workpiece are typically moved relative to each other so that the coating may be applied in layers over the entire surface of the workpiece. A suitable number of layers normal to the surface are deposited for achieving a desired thickness of the coating.
One problem experienced with jet engine components which are plasma spray coated with TBC is the high susceptibility of edge delamination of the TBC during grinding in the manufacturing process. And, TBC coated components are also subject to undesirable thermal shock damage in the gas turbine engine. Spallation is a condition wherein the TBC delaminates over time decreasing the effective useful life of the coated component.
These problems are attributed to the typical process of air plasma spraying the TBC during manufacture which requires that overlapping layers of the deposited coating meet the required thickness levels thereof. As coating thickness is increased, delamination problems correspondingly increase. Accordingly, it is desired to provide an improved method and apparatus for depositing TBC atop metal components for improving the useful life thereof, and alleviating these problems.